1. Field of the Invention
The present invention relates to an ultrasonographic apparatus for diagnosis and, more particularly, to a shifting focus type and/or variable aperture type ultrasonographic apparatus for diagnosis.
2. Description of the Related Art
A diagnostic ultrasonographic apparatus is generally constructed and arranged such that an ultrasonic wave is focused at various predetermined depths of an object such as a human body, thereby enhancing the resolution of an ultrasonic echo image in the lateral direction. A linear array electronic scanning type diagnostic ultrasonographic apparatus, for example, includes a plurality of ultrasonic transducers which are arranged in a linear configuration. It is a common practice with this type of apparatus to achieve a focus effect electronically by delaying transmit wave signals adapted to drive a plurality of ultrasonic transducers relative to each other in one transmission and reception time, i.e., at one rate for transmitting and receiving an ultrasonic wave in the form of a single pulse, the transducers each being associated with a different channel. Another approach to achieve such an electronic focus effect known in the art is delaying received wave signals which are outputted in association with echoes incident to the individual ultrasonic transducers from an object relative to each other and then combining the delayed signals. More specifically, both the two approaches hitherto employed give relative phase differences to transmit wave signals or received echo signals.
The electronic focus system stated above is advantageous in that the focus can be changed rapidly by electronic control and therefore shifted to a plurality of depths on a real time basis. On the other hand, a shifting focus system which shifts the focus in one reception period suffers from two different problems as follows. First, when the taps of delay devices which are adapted to apply different delays to the transducers channels are changed over by switches, an all OFF state causes a dark line to appear at a corresponding point of an image even though the duration of the all OFF state may be short. Conversely, an all ON state (overlap) causes a bright line to appear. Second, even if both of the all OFF state and the all ON state are eliminated, i.e., even if an arrangement is so made as to hold either one of the switches associated with the different regions in an ON state without fail, a difference in picture quality before and after the changeover sharply appears because the ultrasonic acoustic field is different between nearby focus ranges. In this manner, there is a fear that a portion where the changeover has occurred is clearly visible in an output image.
As exemplarily shown in FIG. 4, assume that the focus is shifted from a focus region A defined at a depth of 30 millimeters of an object to a focus region B defined at a depth of 60 millimeters. Then, in a portion where the focus regions A and B overlap each other due to the shift of the focus, the sensitivity is changed to produce a light or a dark strip 12 in an output image. Such a bright line and a light (or dark) strip prevent an output image from appearing even and smooth. This is particularly critical when it comes to diagnosis of the kind deciding normalcy/abnormalcy of a human body by observing delicate changes in the intensity of echoes in the affected part. It is therefore necessary to minimize the influence of a change in sensitivity in the event of a shift of the focus.
To compensate for the change in sensitivity, the switches may be replaced with variable resistor elements which allow the transition from an ON state to an OFF state and vice versa to proceed little by little. This scheme, however, needs an extra implementation for uniformizing the rates of resistance changes in the two focus regions because the variable resistor elements each being associated with a respective one of the focus regions are not exactly the same with respect to characteristics. Such an implementation not only complicates the construction of a control circuit but also needs extra production stages for the selection of elements and adjustment.
The problem discussed above has also been encountered with a variable aperture type ultrasonographic apparatus for diagnosis which receives a reflection from an object while sequentially increasing its effective ultrasound receiving area with the depth. Specifically, when a sectional image of an object received through a comparatively narrow aperture by intermediate transducers of an ultrasonic transducer array and a sectional image received through a comparatively wide aperture by all of the transducers of the array are switched to each other, the sectional image appearing on a video monitor suffers from a discontinuous area between the two regions.